This disclosure concerns techniques and devices for reading the identification code values of multiple radio frequency identification (RFID) tags that are present at the same time in a reading zone. A shielding enclosure confines and concentrates electromagnetic signals propagating to and from multi-read RFID tags that are present in the reading zone, while minimizing interference nulls and shadowing effects.
The RFID tags receive and respond to signals from a reading apparatus, through antennas directed into the shielding enclosure. In some arrangements, a shielding enclosure is arranged to concentrate signal strength in a limited zone for obtaining a robust response, while minimizing signal leakage from the enclosure that could result in unintended responses from RFID tags that are not located in the reading zone. In some arrangements, the shielding enclosure is structured to substantially isolate the reading zone and the antennas by electromagnetic shielding foyer structures and/or by movable shielding doors or covers that can be closed during reading. In further arrangements, the geometry of the enclosure and the orientation of the antennas are configured to provide for numerous signal paths, especially using cylindrical shapes to obtain signal propagation paths that are reflected internally to pass through the reading zone multiple times along signal propagation paths at different angles of incidence. In a reading zone bounded by cylindrical conductive walls for shielding and reflection, this effect is enhanced by tilting the antennas relative to a directly radial orientation.
RFID tags are used to affix binary codes to articles or places or persons, typically with an identifying code value that has at least a portion that is unique to the tag. The code value is associated with one or more records in a database or other data processing system and is used to access information about the tag or the articles, places or persons to which the tag is assigned. Information about the tagged article may be stored in a local inventory handling system or stored remotely and made accessible by lookup operations associated with the RFID code value or portions of the code value. Other portions of the code value may contain digits or bytes or bits that represent a description or classification or status that may not be unique. In a multi-read process, the tag code values of each tag are unique so that each tag can respond during a polling procedure in a manner that signals presence of the tag, and therefore presence of the item to which the RFID tag is attached or the person assigned to carry the tag.
RFID tags are available in different configurations, that respond to different excitation frequencies and produce responses that can be distinguished dependably at different ranges. Transceiver antennas excite or illuminate the tags and sense the response of the tag at one or more frequencies. Some RFID tag and tag code reading configurations are low powered and have a short read range. With some RFID tag and reader configurations, it is advantageous to expose only a single RFID tag to the transceiver antenna at any given time. This enables individual tag codes to be discerned without signaling collisions (namely situations where response signals from two or more tags interfere with one another). Short range RFID tags are useful in personal ID cards, in devices for authorizing single file entry to premises, for metering use of services, etc. The effective reading range or distance is small compared to the size of a person or an article bearing the tag.
In other RFID applications, the RFID tags are affixed to articles that may be in close proximity. For articles such as garments handled in cleaning processes, for example, a number of garments may be bundled or stacked in a manner that placed there RFID tags close together. One technique for discerning RFID code values on garments is to separate the tagged garments from a bundle and to toss of otherwise feed individual garments serially one after another through a chute or along a path where an RFID antenna is mounted and coupled to a controller. Stacked items such as tagged file folders, library books and finished (cleaned) garments might be separated from one another and passed in front of a reader on a conveyor, or a portable RFID reader can be passed in front of the item. Such applications are single-read operations that are possible when it is convenient to bring one tagged item into range of a reader at one time.
U.S. Pat. No. 7,876,220—Aldridge, for example, discloses a garment cleaning and tracking system. The garments are handled in batches for route collection, processing, sorting and delivery. For such applications, it is useful to have a longer reading range. It is also useful to employ a multi-read system capable of resolving collisions (contemporaneous responses) from two or more tags in a process that determines the tag code values of multiple RFID tags present at the same time. U.S. Pat. No. 7,429,912—Tanaka discloses an example of a multi-read tag technique. A transceiver apparatus has an interrogator function during which tags are polled such that tags having a particular value respond to the interrogation and other tags do not. In some systems of this type, a tag that responds (or not) can be disabled from responding again for a predetermined time, allowing other tags to be polled for other values. The code values of multiple tags that are present can be determined by collecting responses of tags at more and less significant bit positions, and disabling selected tags when a collision occurs. The interrogation by polling proceeds bit by bit through a sequence until the code values have been collected for all the tags that are within range of the transceiver. If a closed list of possible code values is known, a transceiver also can inquire for a response at each code value in turn. A system capable of discriminating the codes on plural RFID tags in range of the transceiver is generally termed a multi-read system.
A multi-read tag known as a UHF tag is currently available, operating at 860-960 MHz and having an effective read/response range between the RFID tag and the antennas that emit or receive signals up to a distance that is much larger than the tags or the articles to which the tags are attached. UHF RFID tags respond to received signals (inquiry signals) at relatively low power and emit responses (reply signals) that can be detected at long range, e.g., about 30 feet. Different frequency bands within the range of 860-960 MHz are used in different countries.
It may seem advantageous to use long range tags that operate at low power levels. But there is an operational drawback to having tags response to low power inquiry signals and emit reply signals detectable at a distance that may be larger than the size of a containment holding multi-read tags. As a practical matter, long range and limited power requirements increase the incidence of inadvertently reading RFID tag codes on articles that are not members of the group of tagged articles that are intended to be read when operating the multi-read apparatus. Such systems are prone to read tags on articles that inadvertently are within range of a transceiver, but are not members of the group of articles for which the operator intends to read the RFID codes. In an inventory control process, for example for cleaning plural articles carried in hampers, it is important not only to detect RFID code values, but also to assure, if possible, that all the articles whose code values are read are in fact located in the hamper together with the other articles whose code values are read. Without such assurance, articles thought to be in a group entering a process may actually never have been in the group. Inventory control exceptions occur, such as articles that appear to be missing when the group is checked for completeness when leaving the process.
It is not possible to rely on the tags and readers having a maximum reading distance according to a specification because the distance is only one factor among several factors affecting tag readability. High frequency signals can be transmitted and received over a direct line of sight, or the transmission path may include one or more reflections. Some surfaces reflect signals more efficiently than others. There may be interference patterns where standing waves produce maximum power peaks and minimum power nulls. As a result, the boundaries are not always well defined of the zone in which tags are being read effectively. The system designer has tradeoffs to consider. The RF excitement power level can be made high and the reception can be made sensitive, but RFID tags that are outside the reading zone may be read. The power level and reception sensitivity can be lower, but some tags inside the reading zone may not be read successfully.
In an inventory control system, an object is to account for articles that enter and exit a particular premises or process. If an RFID tag has been detected that was not a member of a group of associated articles that entered inventory, inventory assessments can be erroneous. In the case of multi-read detection of a group of RFID codes in a process involving successive groups, the erroneously detected tag may be a member of a subsequent group. It is not possible accurately to resolve the identities of articles exiting processing steps with the articles that entered those steps, if the entering article list included some article identifications that were not actually in the group.
What is needed is a multi-read system configured so that the steps of reading RFID codes on articles in a group is as sure and dependable as possible. The system should have capacity to handling a group of numerous articles, and perhaps numerous articles that are of relatively large dimensions. These objectives suggest a large reading installation applying high powered polling signals and reading from high gain receiving antennas. However, it is also important surely and dependably to eliminate reading of RFID codes on articles that are not physically in the group being handled as a unit, which suggest that the application of polling signals should be limited. A technique to resolve these objects and aspects is needed.